1. Object of the Invention
This invention relates to liquid ink recording devices. In particular, this invention relates to controlling the print mode of thermal ink jet printing device based on temperature of the printhead and density of the printed image.
2. Description of Related Art
In liquid ink recording apparatuses, an image is formed on a substrate by depositing wet ink on the substrate in a predetermined pattern. One type of liquid ink printing apparatus is a thermal ink jet printer, which utilizes a printhead having a plurality of aligned nozzles that eject ink droplets onto the recording medium. Thermal ink jet devices are designed to give the optimum ink dot size at room temperature. However, as the ambient temperature increases, the ink dot size begins to grow causing adjacent ink drops to overlap. Overlapping of still wet ink dots causes image degradation problems such as bleeding and misting and creates an image that is excessively bold. Further, at higher temperatures, the ink jets tend to ingest air that causes intermittent firing of the jets, which also affects the quality of the image In particular, misfiring leads to a grainy appearance of the image within the solid fill regions. Therefore, it is desirable to maintain a constant drop size by reducing the ink drop size at elevated temperatures to obtain a clear and accurate image.
One method for reducing the drop size is to operate the ink jet printhead in a checkerboard printing mode that utilizes two passes of the printhead while ejecting the required dots in an alternating pattern for each swath of printing. Under this mode for example, when printing left to right, the jets fire in an alternating odd, even, odd etc. pattern and, when printing right to left, the jets fire in an alternating even, odd, even etc. pattern, thus firing every other jet for each pass of the printhead across the printing medium. The benefits to using the checkerboard printing include allowing an ink jet twice as long to refill since each jet is only required to fire at every other dot column. Also, firing every other ink jet in this manner cuts the ink supply demand through the cartridge in half. The additional refill time and reduced ink supply demand reduces misfirings. Further, since diagonally adjacent pixel areas are deposited in the same pass, there is no overlap of ink dots from adjacent pixel areas when the ink is still flowable. This prevents the dots from blurring. An example of checkerboard dot deposition for liquid ink printing is disclosed in U.S. Pat. No. 4,748,453 to Lin et al., which employs a checkerboard printing mode based on the printing medium to prevent blurring of the image when printed on the substrate having poor ink absorptive properties.
Another reason for choosing a checkerboard printing mode is when the density of the printed image is high thus requiring the deposition of numerous closely spaced dots, which can result in blurring. An example of using the Checkerboard printing mode based on image density is discussed in U.S. Pat. No. 5,237,344 to Tasaki et al. To more accurately predict when the use of checkerboard printing mode is appropriate, both the density of the image and the estimated temperature of the printhead is used in U.S. Pat. No. 4,653,940 to Katsukawa.
Another means for controlling drop size in a liquid ink recording apparatus is to vary the frequency at which the ink droplets are deposited on the substrate. In an ink jet printhead, the frequency can be varied by reducing the ejection frequency of each ink droplet from the printhead or by lowering the scanning speed of the recording head. Several devices that vary the frequency of the ejection of droplets when temperatures are elevated are disclosed in U.S. Pat. No. 5,300,968 to Hawkins, U.S. Pat. No. 5,172,142 to Watanabe et al., and U.S. Pat. No. 5,166,699 to Yano et al.
However, the above solutions to controlling the dot size require complicated and expensive methods to select the appropriate printing mode. None account for both the actual temperature of the printhead and the density simply and inexpensively. For example, several of the above methods controlling dot size involve selecting the printing mode based on the substrate composition or based on certain environmental conditions, such as estimated temperature or humidity. Other methods that control the frequency of the droplet ejection rate are based solely on the density of the printed image and do not account for the problems caused by elevated temperatures. Therefore, there is a need to simply and inexpensively control the dot size to maintain a high quality printed image.